Modern liquid toner electrostatic imaging began with the invention of a new class of toners referred to herein as ElectroInk® (which is a trademark of Indigo, N.V. of The Netherlands). This toner is characterized by its comprising toner particles dispersed in a carrier liquid, where the toner particles are comprised of a core of a polymer with fibrous extensions extending from the core. When the toner particles are dispersed in the carrier liquid in a low concentration, the particles remain separate. When the toner develops an electrostatic image the concentration of toner particles increases and the fibrous extensions interlock. A large number of patents and patent applications are directed toward this type of toner and charge directors which are comprised in it. These include: U.S. Pat. Nos. 4,794,651; 4,842,974; 5,047,306; 5,407,307; 5,192,638; 5,208,130; 5,225,306; 5,264,312; 5,266,435; 5,286,593; 5,300,390; 5,346,796; 5,407,771; 5,554;476; 5,655,194; 5,792,584 and 5,5923,929 and PCT Patent publication WO/92/17823, the disclosures of all of which are incorporated herein by reference.
It has been discovered that this type of toner allows for high quality offset printing at high speed. However, this type of printing is described inter alia in patents and patent application numbers 4,678,317; 4,860,924; 4,980,259; 4,985,732; 5,028,964; 5,034,778; 5,047;808; 5,078,504; 5,117,263; 5,148,222; 5,157,238; 5,166,734; 5,208,130; 5,231,454; 5,255,058; 5,266,435; 5,268,687; 5,270,776; 5,276,492; 5,278,615; 5,280,326; 5,286,948; 5,289,238; 5,315,321; 5,335,054; 5,337,131; 5,376,491; 5,380,611; 5,426,491; 5,436,706; 5,497,222; 5,508,790; 5,527,652; 5,552,875; 5,555,185; 5,557,376; 5,558,970; 5,570,193; the disclosures of which are incorporated herein by reference. Systems incorporating various ones of these patents are sold under the names E-Print 1000®, Ominius®, Turbostream™ and Cardpress™.
In general, ElectroInk comprises a polymer or polymers (usually pigmented) which solvate the carrier liquid at some temperature above room temperature (and preferably above normal storage temperatures of 30-40° C.) and do not solvate the carrier liquid or dissolve substantial amounts of it below that temperature. Above the solvation temperature the polymer adsorbs the carrier liquid and is plasticized and softened by it. At elevated temperatures the toner material is thus soft enough to bond with a paper substrate. In practice, the temperature and pressure at which transfer to paper is made are controlled so that the transfer is complete, the transferred toner is fixed to the paper and the image is not squashed.
U.S. Pat. No. 5,908,729, the disclosure of which is incorporated by reference, describes, inter alia, a fluorescent toner, i.e., charged toner particles dispersed in a carrier liquid, where the toner particles are colored with a particulate fluorescent pigment. However, the disclosed toner is not of the type described above. Rather, the pigment is mixed with a low density polyethylene in a planetary mixer. The toner thus formed is not fibrous, but rather is in the form of the particulate material coated with the polyethylene.